Quenching of metals



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CROSS REFERENCE.

UMYIIIIII- Patented Aug. 24, 1943 UNITED STATES PATENT OFFICE QUENCHING OF METALS No Drawing. Application January 12, 1942, Serial No. 426,536

4 Claims.

This invention relates to the quenching of metals; and it is particularly concerned with improved quenching oil compositions comprising a light mineral quenching oil and an oil-soluble thickened vegetable oil in a controlled amount adequate to produce a quenching oil composition having an initial five second quenching speed of at least about 22.0 per cent, without substantially modifying the stress-reducing characteristics of the oil; and it further comprises an improved method of quenching metals wherein the preheated metal is quenched by immersion in a quenching oil bath consisting essentially of a mineral quenching oil and a controlled amount of an oil-soluble thickened vegetable oil adequate to impart to said bath an initial five secnd quenching speed of at least about 22.0 per cent without substantially modifying the stressreducing characteristics of the oil; all as more fully hereinafter set forth and as claimed.

While our invention is useful in the quenching of any metal which is advantageously quenched in an oil bath having a high initial quenching speed, it is particularly useful in the quenching of iron base alloys and it will be described hereinafter in detail in connection with the quenching of steel.

Many metal alloys, particularly iron base alloys, such as carbon steels and alloy steels, require heat treatment for the development of maximum properties of hardness and strength. These properties are dependent upon the establishment of certain physical structures in the metal components of the alloy. In steel, hardenability is determined by the extent to which a martensitic structure is established in the alloy. The production of this structure in steel is usually accomplished by arresting at the desired point, the changes in the internal structure of the alloy which take place during the cooling of the steel from high temperatures. The fact that these physical changes require time for their completion makes it possible to arrest them at the desired point by suitable quick cooling.

Quenching of the steel in aqueous or oil quenching baths is generally used to arrest these physical changes. It is advantageously carried out in such manner that the physical changes in the steel are arrested at or near the point at which maximum hardness is obtained, and is then followed by a tempering treatment involving heating at relatively low temperatures to impart the desired ductility or toughness to the metal at some sacrifice in hardness.

Quenching in aqueous quenching media develops the properties of strength and hardness to the maximum obtainable for a given section of metal. However, quenching in aqueous quenching media is undesirable in many cases because these media tend to set up excessive amounts of internal stress in the steel resulting in distortion and warping and, in the extreme case, cracking of the quenched piece. As a result, aqueous quenching media have been supplanted by mineral oil quenching media where such mineral oil quenching media have sufficiently high quenching speeds to produce the desired properties in the quenched metal because such mineral oil quenching media are particularly adapted to minimize internal stresses and distortion in the quenched product. This results from the fact that the quenching speeds of mineral oil quenching media are substantially slower than those of aqueous quenching media, due to considerable extent to the formation of a more or less persistent vapor envelop about the quenched piece at the beginning of the quenching cycle, and the fact that in the later stages of the quenching cycle the quenching speeds of mineral oil quenching media are sufficiently slow so that the internal stresses developed in the metal in the early stages of the cycle tend to be relieved. However, since the overall cooling efficiency of the mineral oil quenching media heretofore known is not as great as that of aqueous quenching media, it has been diflicult or impossible to effect in mineral oil quenching baths quenching of pieces formed of steels hav ing high critical cooling rates, sufficiently rapidly to develop the properties of strength and hardness to their maximum values, or the quenching of pieces having high mass to surface ratios, sufficiently rapidly to develop the properties of hardness and strength to the maximum depth.

It is desirable therefore to improve the quenching of metals in mineral oil quenching media so that the cooling efilciency in the early stages of the quenching cycle is increased to more nearly approach the cooling efficiency of aqueous quenching media without increasing the quenching speed in the later stages of the cycle sufiiciently to deleteriously afiect the normal stressrelieving properties of the mineral oil.

It is an object achieved by the present invention to provide a method of quenching metals, particularly iron base alloys, in mineral oil quenching media having quenching speeds greater than the mineral oil quenching media heretofore known. It is a further object achieved by the present invention to provide mineral oil quenching compositions having cooling efficiencies more nearly approaching the cooling efficiency of Water while retaining the stress-preventing characteristics of oil quenches.

The critical cooling rate of a metal is defined as the lowest cooling rate at which maximum hardness is developed. For steel the critical cooling rate is the lowest cooling rate which will produce a full martensitic structure in the steel. Thus it is generally most desirable to effect initial quenching at a speed sufficient to produce a cooling rate at least equal to the critical cooling rate for as great a depth in the metal as possible and to effect the quenching with a minimum of distortion.

The cooling efficiency of a quenching medium is customarily defined in terms of quenching speed and the initial five second quenching speed is a convenient measure of this property of quenching oil compositions, which is the property with which this invention is most concerned. The initial five second quenching speed of a quenching oil composition as referred to throughout this specification and in the appended claims is determined according to the following formula:

Temperature risgl iscc. tc s t lcrcentnyaihiblelicairc- Temperature rise in cold quench tcsl. 111mm 111 first {We SPC- in which the respective values are measured as follows:

Cylindrical test pieces one inch in diameter and 2.5 inches long of stainless steel containing about 18.0 per cent of chromium and 8.0 per cent nickel are heated for one hour at 1500 F. in a furnace equipped with an automatic temperature control. Two quarts of the quenching oil composition to be tested are placed in a calorimeter which has a wire screen positioned in the center of the oil bath to insure constant depth of immersion of the samples. The quenching composition is heated to an initial temperature of 100 F.

One of the preheated test pieces is immersed in the quenching bath by means of light tongs for a period of five seconds without agitation. The test piece is then removed and the bath agitated and the maximum temperature reached is measured. A second preheated test piece is immersed in the bath at an initial temperature of 100 F. and the bath is agitated until the maximum rise in temperature has been produced. The maximum temperature rise measured in this cold quench represents the available heat of the test piece. By substituting the values thus determined for the five second quench and for the cold quench in the above formula a measure of the heat removed in the first five seconds as com pared with the total available heat is obtained and is referred to herein as the niitial five second quenching speed.

We have discovered that oil-soluble, thickened vegetable oils have the property of increasing the initial five second quenching speed of mineral oils without lessening their stress-reducing characteristics, and that by incorporating these thickened vegetable oils in suitable quantities in light mineral oils of the type heretofore used in quenching metals, we can produce quenching media having initial five second quenching speeds of at least 22.0 per cent, while retaining the distortion-minimizing characteristics of mineral oil quenching media. We have found further that by quenching pieecs formed of metals having high critical cooling rates or pieces diflicult to quench because of a high ratio of mass to surface, by immersion in quenching oil compositions containing these thickened vegetable oils, We can effect cooling of the metal at a rate more nearly approaching or equal to the critical cooling rate of the metal without creating excessive stresses in the metal. In this way we have been able to develop the hardness of metals having high critical cooling rates to a degree heretofore unattainable with oil quenching media and have been able to effect hardening to a greater depth in pieces having high ratios of mass to surface, and both results can be achieved with a minimum of distortion.

The quenching accelerating agents which we have found suitable for the purpose of our invention are oil-soluble thickened vegetable oils of high molecular weight. These materials have the property, when incorporated in a light mineral oil in suitable amount, of substantially increasing the initial five second quenching speed cf the oil without deleteriously affecting its stress-relieving properties. The efiect of these materials in increasing the initial quenching speed of the oil is due mainly to a decrease in the persistence of the vapor envelop formed when the hot metal is first immersed in the quenching oil bath. By incorporating them in mineral oils such as have been used heretofore for quenching metals, we have found that quenching oil compositions having initial five second quenching speeds of at least about 22.0 per cent may be produced.

Oi the thickened vegetable oils which have been found to be good quenching accelerating agents, we find those oils in which the thickening is due to polymerization of some of the components of the oil are most useful, in that higher initial five second quenching speeds can be obtained with such oils than with those thickened by hydrogenation or oxidation, although the latter types of oils may also be used. Of the thickened vegetable oils we have found a heattreated or thermolyzed tung oil to be a particularly useful quenching accelerating agent. This product is obtained by heating tung oil, for example, in a heating coil to a temperature above about 300 C. so that gelation of the oil is prevented and a thickened liquid product is obtained. Such a thickened tung oil, when incorporated in a mineral oil having a quenching speed of about 17.0 per cent such as has been used heretofore in the quenching of metals, in amounts of about 2.0 to 10.0 produces quenching oil compositions having quenching speeds of about 22.0 to about 355.0 per cent.

The various thickened vegetable oils which we have found suitable do not increase the initial five second quenching speed of a mineral oil in the same degree. Some are more effective than others and they can be used in different amounts. We have found also that in most cases the effectiveness of each thickened vegetable oil increases directly with the amount added up to a maximum and then decreases and the optimum range of concentration for different thickened vegetable oils may differ. In general, however, we have found that amounts between about 1.0 and 20.0 per cent give most satisfactory results. By the use of the proper thickened vegetable oil in suitable amount substantially any initial five second quenching speed between about 22.0 and 35.0 per cent can be obtained. For the purpose of our invention we find quenching oil compositions having initial five second quenching speeds of at least 33.5 per cent are most advantageous because we find that even at these high quenching speeds ciiificulties from distortion are not substantially increased.

In compounding the quenching oil compositions of our invention we select for the quenching bath a mineral oil of the type customarily used for quenching steel and other metals. The oils used for this purpose may be either naphthenic or parafiinic oils and are usually acid treated neutral oils having a viscosity of about 70 to 200 S. U. V. at 100 F., relatively high flash and fire points, and substantial heat stability and resistance to sludging. For optimum results we have found that parafilnic oils which usually have an initial five second quenching speed between about 17.0 and about 19.0 per cent are most advantageous.

Best results are obtained according to our invention by incorporating in the oil an amount of thickened vegetable oil sufficient to produce a quenching oil composition having an initial five second quenching speed which can effect cooling of the metal to be quenched at a rate approaching the rate obtainable with aqueous quenching media. In the case of many steels, initial five second quenching speeds of at least about 22.0 per cent are required for this purpose, and particularly for the quenching of many pieces having high mass to surface ratios these high initial five second quenching speeds are particularly advantageous.

We have found also that mixtures of the thickened vegetable oils of our invention may be used. However, in such case, the effect of the respective agents is not entirely additive and the optimum amounts of the respective agents, when used alone, may produce less satisfactory results in mixtures than do somewhat smaller amounts of each agent.

The ac't'uz'll quenching operation using the quenching compositions of our invention is usually carried out by immersion of the preheated metal in the quenching oil composition until sufficient heat has been removed to reduce its temperature the desired amount. The quenching oil may be used repeatedly with only slight deterioration.

In the following examples there are illustrated specifically several of the more advantageous forms of our invention and the results obtainable thereby.

A series of quenching oil compositions composed of a neutral paraflinic mineral oil having a viscosity of 100 S. U. V. at 100 F. and 2.0 to 10.0 per cent of a calorized tung oil which had been heat treated to thicken it, was made up. The initial five second quenching speeds of the respective compositions were determined as described above and the following results were obtained:

Initial five second quenching speed Quenching oil No. 2 l- 16.98

Quenching oil No. 2+2.0% tung oil 29.44 Quenching oil No. 2+3.0% tung oil 33.46 Quenching oil No. 2+4.0% tung oil 34.24 Quenching oil N0. 2+5.0% tung o1l l. 34.88 Quenching oil No. 2+7.5% tung o1] 34.37 Quenching oil No. 2+l0.0% tung oil 33.20

The quenching accelerating effect of ca'lorized tung oil is demonstrated in these results as well as the fact that the optimum concentration passes through a maximum.

While we have particularly described our invention with reference to certain specific embodiments thereof, it is to be understood that it is not limited to such specific embodiments except as hereinafter defined in the appended claims.

What we claim is:

1. An improved quenching oil composition comprising a mineral quenching oil and a thickened prising a light mineral quenching oil and 3 to 20 per cent by weight of calorized tung oil.

4. A method of quenching metals comprising heating the metal to a temperature above its critical temperature and thereafter cooling the metal by immersion in a quenching oil bath comprising a light mineral oil and a controlled amount of a thickened vegetable oil adequate to impart to the quenching oil bath an initial five second quenching speed of at least about 22.0 per cent.

CLIFFORD G. zoa HORST. BLAINE B. WESCOTI. LESLIE W. VOLLMER. 

